The present invention relates to layered photosensitive members used in electrophotography and, in particular, to improved imaging members.
In the process of electrophotography employing a reusable electrophotographic plate, the plate is uniformily electrostatically charged in darkness and then selectively discharged by exposure to an optical image of the original subject to be copied. The resulting latent electrostatic image develops into a visible image by dusting the electrophotographic plate with finally divided electrostatically attractable toner particles. After the visible toner image is transferred from the plate to a permanent support, such as paper, the plate typically passes through a cleaning station where the remaining toner particles are removed so that the plate may be recycled continuously in the electrophotographic process. Plate cleaning is ordinarily accomplished by means of a soft brush, cleaning web or blade which physically removes the toner particles.
Historically, various electrophotographic photoconductive members have been developed for use for various document reproduction processes. All of these photoconductive elements have their own limitations. For example, a selenium based photoconductive element is not flexible enough for use in sheet like or belt form. Moreover, it is vulnerable to conditions of heat. Zinc oxide based photoconductors are brittle and have low photosensitivity. The development of organic photoconductors such as disclosed in U.S. Pat. No. 3,378,612 to Hoegle et al. overcomes the above disadvantages but these organic photoreceptors have a relatively short use life which is an extreme disadvantage with respect to photoreceptors contemplated for use in high speed automatic electrophotographic machines.
In response to the deficiencies of the aforementioned photoconductors, composite layer photoreceptors containing a photoconductor in another material were developed. One type of composite photoconductive layer is illustrated in U.S. Pat. No. 3,121,006 which describe a number of layers comprising finely divided particles of a photoconductive inorganic compound dispersed in an electrically insulating organic resin binder. The binder material disclosed in this patent comprise a material which is incapable of transporting charge carriers generated by the photoconductive particles for any significant distance. In U.S. Pat. No. 4,265,990 there is disclosed layered photoreceptors having separate photogenerating layers and charge transport layers.
Although the above patents rely upon distinct mechanisms of discharge throughout the photoconductive layer, they generally suffer from common deficiencies in that the photoconductive surface during operation is exposed to the surrounding environment and, particularly in the case of repetitive electrophotographic cycling, these photoconductive layers are susceptible to abrasion, chemical attack, heat and multiple exposure to light. These effects are characterized by a gradual deterioration in the electrical characteristics of the photoconductive layer resulting in the printing out of surface defects and scratches, localized areas of present conductivity which fail to retain an electrostatic charge, and high dark discharge.
As a practical matter, a photoreceptor element must be recycled many times in present commercial electrophotographic processes. Consequently, the imaging layer is subjected to considerable destructive abrasion, especially in the cleaning process. The electrical properties of the photoreceptor layer generally require that it must be extremely thin. Since it is also highly desirable to maintain the uniformity of the photoconductive layer, the photoreceptor is susceptible to deterioration from an abrasion of the photoconductive layer during a web, brush, blade, or other cleaning method in the electrophotographic process. Moreover, when the process requires increased image density by deposition of greater quantities of toner particles on the latent electrostatic image of the photoreceptor, an undesirable increase in background deposits of toner generally results. This, of course, exacerbates the deterioration process because there is greater abrasion required in the cleaning process. These difficulties are particularly acute with organic photoreceptors which have notoriously short use lives because of the physical wear and deterioration inherent in the cleaning and transfersteps of the electrophotographic process.
In response to the longevity problem of organic photoreceptors, it has been found that the use of toner release materials in certain photoreceptors results in photoconductive elements which have a relatively long use life thereby rendering them particularly useful in present day automatic electrophotographic copiers.